1,685 research outputs found
Modeling Premartensitic effects in Ni2MnGa: A Mean Field and Monte Carlo Simulation Study
The degenerate Blume-Emery-Griffiths model for martensitic transformations is
extended by including both structural and magnetic degrees of freedom in order
to elucidate premartensitic effects. Special attention is paied to the effect
of the magnetoelastic coupling in Ni2MnGa. The microscopic model is constructed
and justified based on the analysis of the experimentally observed strain
variables and precursor phenomena. The description includes the (local)
tetragonal distortion, the amplitude of the plane-modulating strain, and the
magnetization. The model is solved by means of mean-field theory and Monte
Carlo simulations. The results show that a variety of premartensitic effects
may appear due to the magnetoelastic coupling. For large values of the
magnetoelastic coupling parameter we find a premartensitic first-order
transition line ending in a critical point. This critical point is responsible
for the existence of large premartensitic fluctuations which manifest as broad
peaks in the specific heat, not always associated with a true phase transition.
The main conclusion is that premartensitic effects result from the interplay
between the softness of the anomalous phonon driving the modulation and the
magnetoelastic coupling. In particular, the premartensitic transition occurs
when such coupling is strong enough to freeze the involved mode phonon.Comment: 28 pages, 15 eps figures, accepted for publication in Phys. Rev.
Determinants of Organisational Structures: An Empirical Study
This article analyses the main factors that determine the organisational structure of a sample of firms located in Catalonia, an autonomous region in the northeast of Spain. The variables studied were identified from among the factors considered in contingency theory and by incorporating elements of the strategic choice approach. After grouping the variables into two factors (related to internationalisation and customer-oriented aspects, respectively) the results revealed three groups of companies according to how they regarded the impact of these factors on organisational structures. In those groups that consider the variables of internationalisation to be modifiers of structure the organisational structures are of the âcomplex classicalâ type, whereas simple forms predominate in the group that believes these variables do not modify their structure.organisational design; contingency; strategic choice.
A new method to determine multi-angular reflectance factor from lightweight multispectral cameras with sky sensor in a target-less workflow applicable to UAV
A new physically based method to estimate hemispheric-directional reflectance
factor (HDRF) from lightweight multispectral cameras that have a downwelling
irradiance sensor is presented. It combines radiometry with photogrammetric
computer vision to derive geometrically and radiometrically accurate data
purely from the images, without requiring reflectance targets or any other
additional information apart from the imagery. The sky sensor orientation is
initially computed using photogrammetric computer vision and revised with a
non-linear regression comprising radiometric and photogrammetry-derived
information. It works for both clear sky and overcast conditions. A
ground-based test acquisition of a Spectralon target observed from different
viewing directions and with different sun positions using a typical
multispectral sensor configuration for clear sky and overcast showed that both
the overall value and the directionality of the reflectance factor as reported
in the literature were well retrieved. An RMSE of 3% for clear sky and up to 5%
for overcast sky was observed
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